1. Field of the Invention
The present invention relates to a blur correction apparatus that corrects blur in a captured image caused by motion (shake) of an image capturing apparatus. The present invention also relates to an image capturing apparatus comprising the blur correction apparatus.
2. Description of the Related Art
Quality degradation of a captured image is mainly caused by motion of the image capturing apparatus called device shake or camera shake during exposure, which is especially problematic due to recent size reductions in image capturing apparatuses or increases in the magnification of a zoom lens. Accordingly, a blur correction apparatus for reducing the influence (blur) of device shake on a captured image has been proposed.
On the other hand, there exists an image capturing method such as follow shot that captures an image while intentionally moving the image capturing apparatus. It is not preferable to apply blur correction without making a distinction between device shake and such an intended movement of an image capturing apparatus. Hence, determining whether the motion of an image capturing apparatus is intentional movement or device shake and correcting the blur correction sensitivity in accordance with the determination result is known.
For example, Japanese Patent No. 3186219 discloses suppressing the response of the blur correction function for the frequency components of panning when the image capturing apparatus is determined to be panning.
For example, a blur correction apparatus mounted on an image capturing apparatus has an arrangement as shown in FIG. 19. In a blur correction apparatus 10, an angular velocity sensor 11 is attached to the image capturing apparatus main body (not shown) to detect its shake as a signal representing an angular velocity. A DC cut filter 12 cuts off the DC (Direct Current) component of the angular velocity signal output from the angular velocity sensor 11, and passes only the AC component, that is, the vibrational component. An amplifier 13 amplifies the angular velocity signal output via the DC cut filter 12, and outputs it. An A/D (Analog/Digital) converter 14 digitizes the angular velocity signal amplified by the amplifier 13, and outputs it.
An HPF (High-Pass Filter) 15, integrator 16, focal length calculation circuit 17, and panning control circuit 18 are implemented by, for example, causing a microcomputer 20 to execute software stored in a nonvolatile memory (not shown).
The HPF 15 cuts off, out of the frequency components of the digital angular velocity signal (angular velocity data) output from the A/D converter 14, low-frequency components equal to or lower than a preset lower cut-off frequency, and outputs high-frequency components higher than the lower cut-off frequency. The integrator 16 integrates the high-frequency components of the angular velocity data output from the HPF 15, and outputs the integration result as angular displacement data. The focal length calculation circuit 17 detects the focal length of the zoom lens provided in the image capturing apparatus (not shown). The focal length calculation circuit 17, for example, acquires the current zoom position of the zoom lens of the image capturing apparatus from a zoom encoder, and computes the focal length (angle of view) of the zoom lens based on the zoom position, thereby detecting the focal length. Based on the focal length and the above-described angular displacement data, the focal length calculation circuit 17 then computes shake correction data for use in the correction of a shake of the optical axis of the image sensor. A blur correction circuit 19 corrects the shake of the optical axis of the image capturing apparatus in accordance with the shake correction data.
The blur correction circuit 19 can be either an optical blur correction circuit which corrects a blur by driving the correction lens in a direction perpendicular to the optical axis and decentering the optical axis, or an electronic blur correction circuit which corrects a blur by moving a region to be read out from the image sensor. Alternatively, it may be a sensor shift blur correction circuit which moves the image sensor in a plane perpendicular to the optical axis.
Based on the angular velocity data output from the A/D converter 14 and the angular displacement data output from the integrator 16, the panning control circuit 18 determines whether the image capturing apparatus is panning (panning determination). More specifically, for example, if the angular velocity data is equal to or more than a predetermined threshold, or if the angular velocity data is less than a predetermined threshold but the angular displacement data (integration result) is equal to or more than a predetermined threshold, the image capturing apparatus is determined to be panning.
The panning control circuit 18 then performs panning control in accordance with the panning determination result. In panning control, first, the lower cut-off frequency of the HPF 15 is gradually raised to reduce the shake frequency domain where blur correction functions. In addition, the value of the time constant to be used for the integral operation of the integrator 16 is gradually made smaller. Accordingly, the blur correction position gradually moves to the center of the moving range so that the value of the angular displacement data output from the integrator 16 gradually approaches the reference value (a possible value in a state without a shake).
On the other hand, upon determining that the image capturing apparatus is not in the panning state, the panning control circuit 18 gradually lowers the lower cut-off frequency of the HPF 15, and gradually increases the value of the time constant to be used for the integral operation of the integrator 16. Accordingly, the lower cut-off frequency of the HPF 15 and the value of the time constant to be used for the integral operation of the integrator 16 return to their initial states so that the panning control is canceled.
For example, Japanese Patent No. 3186219 discloses a method of controlling the HPF 15 and the integrator 16 in the above-described panning mode.
However, the frequency band of the panning operation ranges approximately from DC to 1 Hz, and the frequency band of a camera shake or a body shake ranges approximately from 1 Hz to 10 Hz. That is, the frequency bands are very close. For this reason, the conventional technique disclosed in Japanese Patent No. 3186219 has the following problem.
Upon determining the panning state, the lower cut-off frequency of the HPF 15 and the value of the time constant in the integrator 16 are controlled to increase the signal attenuation amount of the frequency component of panning. At this time, the signal attenuation amount of the frequency component of a camera shake or a body shake during walking also increases. This results in a smaller blur correction effect when the image capturing apparatus is determined to be in the panning state than when it is determined not to be.